I want to change a jpg file of persons photo into vecotr lines and maintain the colors.

Inkscape as a vector editor cannot "break the photo into solid line colors" (Inkscape does not edit the pixel data of bitmap images itself). What Inkscape offers is to trace the imported bitmap image (PNG, JPG, TIFF, GIF, …) based e.g. on brightness of the individual pixels, or their color, and create s single path, or a group of multiple paths, enclosing regions which match the same brightness or color value range. This feature can be accessed via 'Path > Trace Bitmap…'.

A basic introduction to tracing is available in the tutorials that come with Inkscape:
See Inkscape menu 'Help > Tutorials > Inkscape: Tracing'

Additional information about the individual options of 'Trace Bitmap…' can be found in the manual:
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Trace.html>

Alternatively - depending on the original bitmap image and the intent for the vector drawing based on it - one can trace the characteristic shapes of the image manually using the path tools (pencil, pen tool), and then enhance the manually traced drawing by adding gradients and filters for effects like e.g. shading. This works best if the image is imported onto a separate layer, stacked at the bottom and locked, and the paths are drawn on separate layer(s) stacked on top of it.

See also:
FAQ #894: “Converting bitmap (jpg, png...) to vectors (svg)”.